1. Field of the Invention
The invention relates to a hydraulically damped mounting device. Such a device usually has a pair of chambers for hydraulic fluid, connected by a suitable passageway, and damping is achieved due to the flow of fluid through that passageway.
2. Background of the Invention
EP-A-0115417 and EP-A-0172700 discuss two different types of hydraulically damped mounting devices for damping vibration between two parts of a piece of machinery, e.g. a car engine and a chassis. EP-A-0115417 disclosed various “cup and boss” type of mounting devices, in which a “boss”, forming one anchor part to which one of the pieces of machinery is connected, is itself connected via a deformable (normally resilient) wall to the mouth of a “cup”, which is attached to the other piece of machinery and forms another anchor part. The cup and the resilient wall then define a working chamber for hydraulic fluid, which is connected to a compensation chamber by a passageway (usually elongate) which provides the damping function.
In EP-A-0172700 the mounting devices disclosed are of the “bush” type. In this type of mounting device, the anchor part for one part of the vibrating machinery is in the form of a hollow sleeve with the outer anchor part in the form of a rod or tube extending approximately centrally and coaxially of the sleeve. In EP-A-0172700 the tubular anchor part is connected to the sleeve by resilient walls, which define one of the chambers in the sleeve. The chamber is connected via a passageway to a second chamber bounded at least in part by a bellows wall which is effectively freely deformable so that it can compensate for fluid movement through the passageway without itself resisting that fluid movement.
In the hydraulically damped mounting devices disclosed in the specifications discussed above, there is a single passageway. It is also known, from other hydraulically damped mounting devices, to provide a plurality of independent passageways linking the chambers for hydraulic fluid.
In EP-A-0115417, the compensation chamber is separated from the working chamber by a rigid partition, which contains a flexible diaphragm which is in direct contact with the hydraulic fluid and, together with the partition, forms a gas pocket. The diaphragm is configured to give a specific influence on the vibration characteristics of the hydraulically damped mounting device. Those characteristics depend on the stiffness of the diaphragm, by which is meant the change in applied pressure needed to cause unit change in the volume displaced by the diaphragm. Furthermore, the surface of the diaphragm which is in contact with the fluid in the working chamber may be covered by a snubber plate, with openings therein for fluid communication therethrough between the upper surface of the diaphragm and the rest of the working chamber, and it has been found that the size of those openings also affects the characteristics of the mount.
In GB-A-2282430, a mounting device is disclosed of the “cup and boss” type, with two diaphragms. The two diaphragms are arranged to have different characteristics, such as different stiffnesses or different effective stiffnesses, due to the shape of the openings by which fluid reaches those diaphragm parts from the working chamber. GB-A-2282430 also discloses that either or both of the diaphragms may be convoluted.
It is also known to provide an additional passageway to link the working chamber with another hydraulic chamber, separate from the compensation chamber, the additional passageway having a lower fluid resistance than the passageway between the working and compensation chambers.
In U.S. Pat. No. 5,180,148, a passage is formed between a pressure receiving chamber and a second equilibrium chamber. The passage is normally closed, and is held closed by the action of an elastic dish member which bears against a diaphragm via a thin rigid disk. The passage can be opened by application of a vacuum on the side of the elastic dish member not bearing against the diaphragm, so that the elastic dish member is separated from the diaphragm.
In U.S. Pat. No. 6,017,024, a passage is formed between a primary fluid chamber and an auxiliary fluid chamber. The passage is normally held closed by the action of a tensile spring, which bears against a circular metal disk. The metal disk is movable relative to the passage by flexing of a surrounding annular rubber member. This movement is controlled by control of the pressure in a vacuum chamber underneath the metal disk.
The additional passageway is typically known as a bypass or secondary channel. Normally, the bypass channel is opened, e.g. by supplying a vacuum to actuate a switching member, when the engine is at idle. The lower fluid resistance that occurs when the bypass channel is open reduces the dynamic mass effect of the body of hydraulic fluid contained in the channels, and thereby increases the frequency of the eigenmode exhibited by the device. This is advantageous in that it enables a stiffness drop or dip to be positioned at the main engine forcing or problem frequency, typically the firing frequency. The reduction in stiffness results in better isolation at idle and reduction of vibration in the vehicle cabin.
When the vehicle is in ride mode it is important that the bypass channel remains closed in order to keep the main passageway tuned to a lower frequency commensurate with its primary function of damping overall engine-gearbox vibration. This means the vacuum switched valve has to be capable of resisting significant pressures, e.g. when the vehicle drives over rough surfaces and/or when the mounting is subject to large movements.